Some of the most problematic aspects of Digital Subscriber Line (DSL) communications are faults including but not limited to the following:
The presence of unfiltered devices connected to the telephone line such as telephones, answering machines, fax machines and in general and device connected to the telephone line. Many of these devices impose nonlinear behavior on transmitted signals. Harmonic frequencies arising from these nonlinearities generate a disturbance that is often the dominant noise source limiting data rates and loop reach.
The presence of impulsive noise.
A malfunctioning modem on either or both ends of the line.
Unlike other faults such as high crosstalk, AM ingress, and bridged taps, the above faults are not always identifiable by doing a separate analyses of the attenuation (HLOG) and quiet line noise (QLN) vectors as a function of frequency (per subchannel) that are available as a part of standard-compliant DSL links.
For example, high crosstalk or AM can be identified by analyzing QLN, and bridged taps can be identified by analyzing HLOG. Analyzing these vectors separately in isolation cannot always uncover problems such as nonlinear effects (caused by unfiltered devices), intermittent effects that occur when the measurement is not in progress (such as impulsive noise), or problems with the modems themselves.